A method for measuring the speed of an EC motor, which is provided for operating a hydraulic pressure medium transmitter, is described in DE 102 32 500 A1, where the pressure medium transmitter adjusts a clutch, via which a torque is transmissible from the internal combustion engine to the transmission of a motor vehicle. While the secondary part of the EC motor is moved with respect to the primary part by commutating the current supply to the winding, the position of the magnet segments of the secondary part with respect to the primary part is detected using Hall sensors. The position measuring signal changes its value after each segment, i.e., rotational angle, of 6°. The position measuring signal is differentiated to form the rotational speed signal. To improve the resolution of the speed signal over the periphery of the EC motor, the segment between two sensor signals is subdivided into a number of substeps. The system is synchronized at the position of a sensor signal in such a way that the instantaneous rotational speed is known. The rotational speed from substep to substep is computed starting from this point using the model until synchronization is performed again at the next sensor signal. The resolution of the rotational speed signal over the periphery can be improved using this method.
However, since the magnet segments have tolerances regarding their positioning on the periphery of the secondary part and regarding their dimensions in the circumferential direction, which are not individually known in detail, inaccuracies which are not, or at least not fully, compensated even by subdividing the segment between two sensor signals into substeps occur when the model is synchronized. If the angular position between two magnet segment-sensor combinations in the case of an EC motor having four pairs of poles and three Hall sensors, for example, differs from a setpoint value by ±20% and the speed of the EC motor is 1000 rpm, the position measuring signal changes value every 2.5 ms on the average, i.e., from 2 ms to 3 ms. The speed thus varies from 833 rpm to 1250 rpm, i.e., it may differ from the previous value by approximately 40%. These sudden changes in the measured speed, exhibiting differences of ±20% from the actual speed, considerably affect a speed governor, for example, or only allow low-dynamics regulation.
While the speed signal could be smoothed, for example, by forming the mean of the next-to-last, last, and present measured speed values, the averaged measured speed value would not reflect the speed at the present point in time, but at the time of the last change in the position measuring signal.